The Study Of DCT-based Public And Private Watermarking

As a potential and effective way to protect the copyright of digital products, digital watermarking becomes a very active research area of signal and information processing. After deeply analyzing the technical characters and request of image watermarking, two present algorithms are improved from the point of enhancing their imperceptibility and robustness, and two DCT-based image watermarking methods which can be applied to copyright protection are discussed in this paper.The first discussed method is an improved DCT-based private watermarking algorithm. In selecting the lower frequency coefficients of DCT, the DC component is excluded, because DC component is far higher than other lower frequency coefficients, and Human Visual System is more sensitive to the change of DC component. In order to improve the reliability of correlation test, the watermark sequences are composed of random numbers drawn from Gaussian (N (0,1)) distribution which is produced by Box-Muller method, and the sequences can be easily reproduced by providing to the generating algorithm the correct seed (hey). To optimize the algorithm further, the watermarked image is visually masked based on the characteristics of Human Visual System (HVS). In the visual masking, a modified coefficient is used to increase the robustness of the watermarking.The second discussed method is a DCT-based public watermarking (blind watermarking) algorithm. As it is capable of revealing the mark presence without comparing the marked and original images, this algorithm is more convenient and more preferable. On the basis of statistical analysis, two formulas are presented. One is used to calculate the correlation between the DCT coefficients of image to be detected and the inserted watermark. The other is used to calculate the threshold of the correlation. The method of inserting watermark in color image is discussed on the basis of grayscale image.Finally, in order to test the imperceptibility and robustness of the two improved algorithms, a lot of experiments were done. The experimental results indicate that: the improved DCT-based private watermarking method is more imperceptible and robust than the original algorithm; The provided public watermarking method is simple, convenient, and has a wide application prospect, though its robustness is a little lower than the private watermarking algorithm's.The research of this paper is occupying a forward position in the relevant field. It has important theoretical value and application prospect. The provided methods and experiment results in this paper have important reference value both in further studying of digital watermarking and in developing relevant computer software.